Field of the Invention
The present invention relates to an image processing apparatus, a method of controlling the same, and a storage medium.
Description of the Related Art
Conventionally, image forming apparatuses that adopt an electrophotographic method form an image by exposing a photosensitive drum with a laser beam modulated according to an image signal, or the like so as to form a latent image, developing the latent image using toner, which is a charged color material, and transferring and fixing the developed toner image onto a sheet. In each pixel, such image forming apparatuses form an image at low gradation, and thus halftone processing for expressing gradation using binary pixels is adopted for multi-value image data that expresses multi-gradation in each pixel, in order to reproduce the halftone of image data in a stable manner with fidelity. Dithering and an error-diffusion method are used in this halftone processing. In dithering, a threshold value table called dither matrix is used, and pixel values of image data and corresponding threshold values of the dither matrix are compared. By setting target pixels to a value indicating a laser beam being on (light-on) or a value indicating a laser beam being off (light-out) based on this comparison, a pattern of dots is formed so as to express gradation.
In addition, such image forming apparatuses perform density adjustment in order to express ideal gradation. In this density adjustment, halftone processing is performed on image data (patch) that has not undergone density adjustment and is composed of a plurality of different pixel values, and a test chart constituted by patch patterns of dots is generated. The generated test chart is then printed, and the densities of the patches of the printed test chart are measured. Subsequently, the density characteristics for the current pixel value are calculated from the measured density values, and a density correction table for converting input pixel values into pixel values that are indicated by the current density characteristics, which are target densities for the input pixel values, is generated from the calculated density characteristics. Density adjustment is then performed by converting the input pixel values using the density correction table (for example, see Japanese Patent Laid-Open No. H11-98357).
Such density adjustment is required to be performed on all the pixels that have undergone halftone processing, and pixels with a resolution at which output is performed. Therefore, the test chart is printed for each resolution and a density correction table is generated, or a density correction table for a resolution is calculated from a density correction table of halftone processing for other similar density characteristics, through approximation. This is described in Japanese Patent Laid-Open No. 2009-232455, for example.
In the above-described density adjustment, it is necessary to print a test chart and to generate a density correction table, for each resolution at which the printing apparatus performs printing. Therefore, in the case of an image forming apparatus that has a printing mode for a plurality of resolutions, it is necessary to perform an operation of printing a number of test charts corresponding to the number of the resolutions, and generating density correction tables.
In addition, in a case of calculating a density correction table for a resolution, through approximation, from a density correction table for halftone processing for other similar density characteristics, density correction tables corresponding to a plurality of resolutions can be generated from one test chart. However, if the density characteristics of the image forming apparatus change due to an environment or temporal change, there is an issue that an error of density characteristics for different resolutions in the density correction table generated in this manner increases, and the accuracy of the density correction table decreases.